1. Field of the Invention: The invention relates to perforating guns employed for perforating the casing and the adjoining production formation of a subterranean well, particularly to a gun wherein the vertically spaced shaped charges are successively fired from the bottommost charge upwardly to the uppermost charge.
2. History of the Prior Art: The utilization of a plurality of vertically and angularly spaced shaped charges to effect the perforation of a subterranean well represents the modern approach to achieving perforation of the well casing and the adjoining production formation. In recent years, it has become desirable to mount the shaped charges of the perforating gun within or upon a tubular housing which in turn is run into the well on the bottom of a tubular string, which may comprise the production string. Such shaped charges are substantially concurrently fired by the detonation of a primer cord which passes successively past the primer ends of each of the shaped charge containers.
It is not uncommon for the zone to be perforated to extend from ten to two thousand feet in length, thus, necessitating the fabrication of the perforating gun as a plurality of axially stacked, substantially identical gun sections. The transmission of the firing energy from the uppermost gun section to the lowermost section is commonly accomplished through the mounting of booster charges on each of the ends of a primer cord which is utilized only within an individual gun section. The booster charges are required to insure that sufficient energy is transmitted from one primer cord to the next to insure the successive ignition of all primer cords. There have been many instances of unsuccessful transfer of ignition energy from one gun section to the next, necessitating the removal of the perforating gun from the well to effect the necessary replacements of primer cords and booster charges to accomplish the firing of the remaining shaped charges.
Additionally, with the assembly of such a large number of individual gun sections, it is difficult to effect a sealed connection of all components of the perforating gun housing containing the gun charges in order that the interior of the housing is isolated from well fluids. As is well known to those skilled in the art, even the most carefully assembled units may, after being run into the well, and particularly a well having substantial deviations from the true vertical, experience stresses which result in the creation of leakage paths for well fluids to enter the perforating gun housing. Since the shaped charges are normally sealed within a container, and the primer cord for activating such shaped charges is encased within a thermoplastic tube, the existence of well fluids within the perforating gun housing will not necessarily prevent the detonation of the primer cord and the subsequent detonation of the shaped charges. The presence of fluids within the gun housing during the detonation of the shaped charges, results in a very substantial hydraulic expansion force being exerted on the wall of the perforating gun housing. In some instances, the expansion is sufficient to bulge the housing into tight engagement with the casing and thus effectively prevent the removal of the perforating gun from the well. It follows that there is a definite need in the subterranean well industry for a method and apparatus for perforating the well casing and the adjoining production formation which will not fire the shaped charges when the lower portions of the perforating gun housing are partially filled with well fluids. Additionally, there is the further requirement that the resulting multisection perforating gun be capable of convenient and safe assembly at the well site, and this can only be accomplished by substantially reducing the number of booster charges that must be assembled to the primer cord by which the shaped charges are to be fired.